1. Technical Field
The present invention relates in general to coil fabrication in thin-film magnetic recording heads, and particularly to producing high resolution coils in thin-film magnetic recording heads using phase-shifting masks.
2. Description of the Related Art
Thin-film magnetic read/write heads are used for magnetically reading and writing information upon a magnetic storage medium such as a magnetic disk or magnetic tape. It is highly desirable to provide a high density of information storage on the magnetic storage medium.
Increased storage density in a recording system may be achieved by providing an area density as high as possible for a given recording surface. In the case of rotating disk drives (both floppy and hard disk), the area density is found by multiplying the number of flux reversals per unit length along the track (linear density in units of flux reversals per inch) by the number of tracks available per unit length in the radial direction (track density in units of tracks per inch).
The demand for increased storage density in magnetic storage media has led to reduced magnetic head dimensions. Magnetic heads are now fabricated in a manner similar to that used for semiconductor integrated circuits in the electronic industry.
During fabrication, many thin-film magnetic heads are deposited across the entire surface of the wafer (or substrate). After the layers are deposited, the wafer is "diced" or sliced into many individual thin-film heads, each carried by a portion of the wafer so that an upper pole tip, a lower pole tip, and a gap are exposed. Pole tips and gap (and the portion of the substrate which underlies them) are then lapped in a direction generally inward, toward the center of the thin-film head, to achieve the desired dimensions. This lapping process is a grinding process in which the exposed portion of top and bottom pole tips and gap are applied to an abrasive, such as a diamond slurry. Electrical contacts are connected to conductive coils. The completed head is next attached to a carrying fixture for use in reading and writing data on a magnetic storage medium such as a computer disk.
FIG. 1 is a diagram of a thin-film head. The thin-film magnetic head 5 consists of a bottom yoke 2 of Permalloy, some insulating layers, a spiral conductor 4 (coil), and a top yoke I of Permalloy, which is joined to the bottom yoke at the back gap but separated from it by a thin insulator at the recording gap. The Permalloy pole tips 6 and 3 at the recording gap are often called pole tip 1 (P1) and pole tip 2 (P2), respectively.
In operation, a magnetic storage medium is placed near the exposed upper and lower pole tips (P1 and P2). During the read operation, the changing magnetic flux of the moving storage medium impresses a changing magnetic flux upon upper and lower pole tips. This magnetic flux is carried through the pole tips and yoke core around the conductor coil 4. The changing magnetic flux induces an electrical voltage across the conductor coil 4 which may be detected using electrical detection circuitry. The electrical voltage is representative of the changing magnetic flux produced by the moving magnetic storage medium.
During a write operation, an electrical current is caused to flow in the conductor coil 4. This electric current induces a magnetic field in top yoke 1 and bottom yoke 2 and causes a magnetic field across the gap between the upper and lower pole tips 3 and 6. A fringe field extends in the vicinity beyond the boundary of the pole tips and into the nearby magnetic storage medium. This fringe field may be used to impress magnetic fields upon the storage medium and write information.
In the manufacture of thin-film magnetic read/write heads, photolithographic printing is employed to fabricate circuit pattern images onto rigid substrates. In this process, photosensitive films called photoresist are coated onto the substrate, exposed to light, and then developed in an alkaline developer solution. Upon development, a pattern configuration forms in the photoresist corresponding to a change in solubility of those regions of the photoresist material exposed to the irradiating light. The clarity or resolution of the lines which define these patterns at microns or even sub-micron geometries to a great extent serves as a limitation to the photolithographic process. Today, photolithographic technology is approaching its ultimate limit, the point beyond which resolution cannot be improved because of diffraction effects, incompatibility of materials, and complexity of processing.
One of the major resolution problems which exists in processes of this type is called reflectivity. This is caused by the fact that some of the light striking a thin layer of photoresist material will usually pass through the layer and be reflected upward from a lower reflective layer during the radiation exposure. The reflectivity problem as it relates to thin-film magnetic heads is illustrated in FIG. 2.
During fabrication, a ferromagnetic Permalloy is deposited on a substrate to create the P1 yoke. Overlaying the P1 yoke is a thin layer of dielectricnonmagnetic material which controls the magnetic gap between pole tips. Overlaying said magnetic gap (usually alumina sputtered material) is a layer I1 of electrically insulating material (usually a high temperature polymerized organic material such as photoresist). The insulation layer I1 cannot fully planarize the step created by the edges of the P1 yoke down to the substrate surface. Prior to the coil photostep, a CrCu seed layer is sputter deposited on top of the insulation layer I1. Overlaying the seed layer is placed a positive photoresist and the coil mask. During the expose cycle of the coil process, the actinic illumination is obliquely reflected from the highly reflective copper seed layer at the step, creating severe notching illumination. Also, the incident light is generally not perfectly normal to the surface of the photoresist layer, and as it may be diffracted upon passage through the photoresist, the incident light will be reflected angularly from the surface of the substrate rather than normally therefrom. Such light will impinge upon the unexposed portions of the photoresist and some may again pass through the photoresist to strike the opaque portions of, for example, a photo mask, and this light will be reflected back into some portions of the photoresist which are not intended to be exposed. This is called "flare" and results in a loss of contrast.
As a result of light being reflected at the seed layer step and other angular light from the reflective surface, there may be a pronounced detrimental effect upon the ultimate resolution which can be obtained upon photo development. This reflective notching phenomenon, experienced when patterning or photo developing the material, could cause inter-Cu coil shorts.
Previous attempts to correct the reflectivity problems, as disclosed in, for example, U.S. Pat. No. 4,102,683, call for interposing a light absorbing layer between the surface of the substrate and the photoresist material. These so-called anti-reflective layers have the property of absorbing light which passes through the photoresist and not reflecting it back upward. They may be comprised of, for example, a quarter-wave plate having an odd multiple thickness of one-quarter of the wavelength of light to which the photoresist layer is sensitive. This plate is comprised of silicon dioxide thermally grown or deposited in any manner, such as by the decomposition of silane. Alternatively, such a light absorbing layer may be manufactured by mixing a fluorescent dye such as an organic phosphor with an organic binder such as Fluorel made by the 3-M Company or Viton made by DuPont, which is a rubber.
However, such prior art anti-reflective layers have exhibited a number of problems not the least of which is the fact that, in general, when the surface of the substrate is irregular, there is required an additional planarization material or layer. Other disadvantages are, for example, that when prior art organic binders have been patterned by a wet etch development, such layers develop isotropically resulting in undercutting during development which results in a narrow processing latitude or even complete lift-off of sub-micron geometries. Also, certain organic binders such as polymethylmethacrylate (PMMA) and polybutene-sulfone when employed as an anti-reflective sublayer have poor stability as vehicles for etching patterns onto the substrate. Moreover, the prior art organic binders cannot be coated in a fully conformable layer at or near a step region. The lack of thickness control will create larger undercuts and variable light reflected from the seed layer at or near the steps region. In addition, another prior art technique is to coat the resist with a top layer of polymer material which acts as an internal anti-reflection coating and minimizes internal (resist) reflections. In all the stated techniques, the actinic radiation that finally acts upon the resist must continue to be of the same intensity to provide for the proper contrast. These techniques only slightly modify the way the radiation is coupled into the resist and in many applications they have to be tailored to each individual situation. Moreover, anti-reflective layers and actinic dyes in the photoresist only reduce the reflective notching problem. They do not eliminate the problem of coil shorts, thus, resulting in a lower thin-film magnetic head yield.
Therefore, it would be desirable to provide a method of fabrication for thin-film magnetic heads which provides a simple and effective method of eliminating notching at the P1 yoke step and improving coil resolution in the photoresist processing step.